Method for separating heavy hydrocarbons from a hydrocarbon-rich fraction

ABSTRACT

Disclosed is a method for separating heavy hydrocarbons, in particular C6+ hydrocarbons and/or aromatic hydrocarbons, such as benzene, from a hydrocarbon-rich fraction to be liquefied, in particular from natural gas, the hydrocarbon-rich fraction being precooled prior to separation of the heavy hydrocarbons. One or a plurality of C5 hydrocarbon-rich fractions are mixed into the hydrocarbon-rich fraction prior to the pre-cooling thereof and/or into the separation column used for separating the heavy hydrocarbons at least in such a quantity, preferably in liquid form, that freezing of the heavy hydrocarbons to be separated is prevented.

The invention relates to a method of separating heavy hydrocarbons, in particular C₆₊-hydrocarbon and/or aromatic hydrocarbons such as benzene, from a hydrocarbon-rich fraction to be liquefied, in particular from natural gas, where the hydrocarbon-rich fraction is precooled before the heavy hydrocarbons are separated off.

Heavy hydrocarbons, hereinafter referred to as HHC, are usually removed from natural gases by means of a separator or an HHC removal column since they would otherwise lead to precipitation of solids in liquefied natural gas (LNG) at low temperatures, If HHC which are virtually insoluble in the liquefied natural gas at low temperatures, e.g. the solubility of benzene is less than 1 molar ppm at a temperature of −162° C., have to be separated off from the natural gas, the prior art provides for removal by distillation, in particular at high natural gas pressures which allow a lower energy consumption for natural gas liquefaction. The natural gas is usually precooled against refrigerants, preferably to from −25 to −55° C., before it is fed into the HHC removal column. The runback necessary for removal of the HHC at the top of the removal column is either produced by means of a refrigerant or a substream of the liquefied natural gas (LNG) is used as runback.

However, the previously described processes can no longer be employed in the case of very light natural gases, which for the purposes of the following text are hydrocarbon-rich fractions or natural gases whose content of C₂₊-hydrocarbons is less than 15 mol %, since

-   -   HHC such as benzene would freeze out during precooling of the         natural gas,     -   HHC such as benzene would freeze out in the HHC removal column,     -   the liquid throughput of a separation stage (e.g. a separation         tray, ordered packing and/or a layer of random packing elements)         required for the hydraulic function of the HHC removal column         cannot be achieved and     -   the density difference between the gaseous and liquid phases         required for the hydraulic function of the HHC removal column         cannot be achieved.

It is an object of the present invention to provide a method of separating heavy hydrocarbons from a hydrocarbon-rich fraction to be liquefied, which, with a moderate outlay in terms of apparatus, allows adequate separation of HHC, in particular benzene, from the hydrocarbon-rich fraction to be liquefied without appreciably increasing the energy consumption for the liquefaction process.

To solve this problem, a method of the type in question for separating heavy hydrocarbons from a hydrocarbon-rich fraction to be liquefied, which is characterized in that a C₅-hydrocarbon-rich fraction is added to the hydrocarbon-rich fraction before the latter is precooled and/or upstream of the removal column serving to separate off the heavy hydrocarbons at least in such an amount, preferably in liquid form, that freezing-out of the heavy hydrocarbons to be separated off is avoided, is proposed.

According to the invention, (a) C₅-hydrocarbon-rich fraction(s) is or are added to or injected into the hydocarbon-rich fraction to be liquefied before the latter is precooled and/or upstream of the removal column serving to separate off the heavy hydrocarbons, with the amount thereof being selected so that freezing-out of the heavy hydrocarbons to he separated off can be effectively avoided in the precooling stage and/or the removal stage for the heavy hydrocarbons downstream of the precooling stage. Here, the C₅-hydrocarbon-rich fraction(s) is/are preferably present in liquid form. For this purpose, the method of the invention requires provision of at least one suitable C₅-hydrocarbon-rich fraction, which will be discussed below.

In contrast to the prior art methods described at the outset, the method of the invention makes sufficient removal of heavy hydrocarbons (HHC), in particular benzene, from very light natural gases possible, so that freezing-out of these components in the liquefaction process can be avoided.

Further advantageous embodiments of the method of the invention for separating heavy hydrocarbons from a hydrocarbon-rich fraction to be liquefied, which represent subjects of the dependent claims, are characterized in that

-   -   the C₅-hydrocarbon-rich fraction(s) to be added is or are at         least partly formed from the hydrocarbon-rich fraction to be         liquefied,     -   the content of C₂₊-hydrocarbons in the hydrocarbon-rich fraction         to be liquefied is not more than 15 mol %,     -   the C₅-hydrocarbon-rich fraction(s) to be added contains or         contain from 50 to 100% by volume of i- and/or n-pentane,     -   the C₅-hydrocabon-rich fraction(s) to be added additionally         contains or can contain propane, butane, hexane and/or higher         hydrocarbons,     -   a substream of the hydrocarbon-rich fraction which has not been         precooled is fed as heating medium into the removal column,     -   the C₅-hydrocarbon-rich fraction fed to the removal column is         cooled to a temperature in the range from −100 to −130° C. and         is preferably fed into the removal column above the feed point         for the hydrocarbon-rich fraction to be liquefied,     -   a substream of the hydrocarbon-rich fraction which has been         freed of heavy hydrocarbons and has been liquefied is fed as         runback into the removal column and/or     -   the hydrocarbon-rich fraction to be liquefied is precooled to a         temperature in the range from −25 to −55° C. before the heavy         hydrocarbons are separated off

The method of the invention for separating heavy hydrocarbons from a hydrocarbon rich fraction to be liquefied and also further advantageous embodiments thereof are illustrated below with the aid of the example shown in FIG. 1.

The hydrocarbon-rich fraction 1 to be liquefied, which is a very light natural gas, is precooled to a temperature in the range from −25 to −55° C. in the heat exchanger E1 and subsequently fed via line 2 to a removal column T1 serving to separate off the heavy hydrocarbons, with the precooled fraction 2 possibly being present in two-phase form.

The cooling of the hydrocabon-rich fraction to be liquefied in the heat exchangers E1, E2 and E2 can be effected against one or more refrigerant circuits and/or refrigerant mixture circuits. Since these are not subject matter of the method of the invention, a detailed presentation is superfluous.

According to the invention, a C₅-hydrocarbon-rich fraction, the origin of which will be explained later, is added via line 15 to the hydrocarbon-rich fraction 1 to be liquefied in at least such an amount that freezing-out of the heavy hydrocarbons to be separated off in the removal column T1 is avoided in the removal column T1 and in the liquefaction stage E2/E3. This C₅-hydrocarbon-rich fraction to be added is preferably present in liquid form.

This C₅-hydrocarbon-rich fraction 15 to be added to the hydrocarbon-rich fraction 1 to be liquefied preferably contains from 50 to 100% by volume of i- and/or n-pentane. Furthermore, it can additionally contain small amounts of propane, butane, hexane and/or higher hydrocarbons. The composition of the C₅-hydrocarbon-rich fraction 15 will be to a first approximation determined by the composition of the hydrocarbon-rich fraction 1 to be liquefied and the heavy hydrocarbons present therein.

In an advantageous embodiment of the method of the invention, a substream of the hydrocarbon-rich fraction which has not been precooled is fed as heating medium via line 1, in which a depressurization valve a is arranged, to the removal column T1. As an alternative to or in addition to this heating, an external heat transfer medium, which is not shown in FIG. 1, can also be employed for heating the removal column T1.

The removal column T1 is, for energy reasons, operated very close to the pressure under which the hydrocarbon-rich fraction 1 to be liquefied is present. The separated-off heavy hydrocarbons are taken off in liquid form from the bottom of the removal column T1 via line 7.

This liquid fraction 7 is fed via the depressurization valve b into a stabilization column T2 whose function will be explained in more detail below.

In the process shown in FIG. 1, a mixture of two different fractions, which will likewise be explained in more detail below, is fed as runback via the lines 14 and 20, in which depressurization valves e and i, respectively, are arranged, into the removal column T1.

At the top of the removal column T1, a hydrocarbon rich fraction which has been freed of heavy hydrocarbons is taken off via line 3 and the main stream 4 thereof is fed to the heat exchanger E2 and in this cooled to a temperature in the range from −110 to −130° C. and liquefied. The liquefied hydrocarbon-rich fraction is fed via line 5 to the heat exchanger E3 in which it is supercooled to a temperature in the range from −150 to −160° C. and subsequently passed via line 6 as liquefied product (LNG) to further use.

A substream of the hydrocarbon-rich fraction 5 liquefied in the heat exchanger E2 is fed via line 19 to the pump reservoir D1. To stabilize the pressure in the pump reservoir D1, a substream of the hydrocarbon-rich fraction 3 which has been freed of heavy hydrocarbons is taken off via line 18 and fed via valve h to the pump reservoir D1. A liquid fraction is taken from the bottom of the pump reservoir D1 and conveyed as runback via line 20 by means of the runback pump P1 via the depressurization valve i into the removal column T1. The runback pump P1 serves to pump the liquid fraction 20 taken off from the pump reservoir D1 at a slightly higher pressure than that prevailing in the removal column T1. Here, pressure regulation of the pump reservoir D1 is effected by means of the above-described substream 18.

As an alternative or in addition, a substream of the refrigerant or refrigerant mixture used for cooling and liquefying the hydrocarbon-rich fraction 1 can also be used as cold supply in order to produce runback in a separate heat exchanger, which is not shown in FIG. 1, at the top of the removal column T1.

As mentioned above, the liquid bottom product 7 from the removal column T1 is fed to the stabilization column T2. While the removal column T1 is preferably operated at pressures in the range from 30 to 60 bar, the stabilization column T2 is preferably operated at pressures in the range from 10 to 30 bar. The bottom product 8 from the stabilization column T2 is a stabilized condensate fraction which can be stored at atmospheric pressure and sold as “gasoline”. A substream 9 of this fraction is warmed in the heat exchanger E against a suitable medium and partially vaporized and serves as bottom heating for the stabilization column T2.

The overhead product 10 from the stabilization column T2 is cooled in the heat exchanger E4 against ambient air, cooling water and/or refrigerant or refrigerant mixture and thus partially condensed. In the runback vessel D2, the remaining gas phase 11 is separated off and passed to a further use, for example as fuel gas for a gas turbine. As an alternative, the overhead product 10 from the stabilization column T2 can also be completely condensed in the heat exchanger E4, so that no residual gas is obtained.

The liquid fraction 12 obtained in the runback vessel D2 is pumped by means of the pump P2 to a pressure which is slightly higher than the pressure of the removal column T1. A first substream of this C₅-hydrocarbon-rich liquid is fed as runback via line 13, in which a depressurization valve c is arranged, to the stabilization column T2, while according to the invention the second substream 14 is, after having been supercooled in the heat exchangers E1 and E2, fed at a temperature in the range from −100 to −130° C. to the removal column T1. Before being fed to the removal column T1, this stream 14 can either be mixed with the above-described runback stream 20 or, as represented by the conduit 14′ represented by the broken line, fed in at a place between the top of the removal column T1 and the feed point for the hydrocarbon-rich fraction 2 to be liquefied.

The amount of the first substream 13 of the C₅-hydrocarbon-rich fraction employed as runback is set so that, the content of heavy hydrocarbons, in particular the benzene content, of the C₅-hydrocarbon-rich fraction is sufficiently low for freezing-out of heavy hydrocarbons, in particular benzene, in the liquefaction process to be avoided.

Depending on the pentane concentration in the hydrocarbon-rich fraction 1 to be liquefied and the pentane losses in the overhead product 3 from the removal column T1, it can be necessary to introduce a small amount of C₅-hydrocarbons into the internal circuit 14 of the C₅-hydrocarbon-rich fraction. This occurs via line 16 and depressurization valve f. During start-up of the liquefaction process, the internal circuit 14 of the C₅-hydrocarbon-rich fraction is built up by introduction of C₅-hydrocarbons via line 16 and depressurization valve f for a limited time.

The addition of a C₅-hydrocarbon-rich fraction to the hydrocarbon-rich fraction 1. to he liquefied, as mentioned at the outset, is preferably effected by taking off a substream 15 from the internal circuit 14 of the C₅-hydrocarbon-rich fraction, with the regulating valve d being provided for regulating the amount, As an alternative or in addition, addition of C₅-hydrocarbons can be realized via the line sections 17 and 15 and regulating valve g.

The liquefaction and supercooling of the hydrocarbon-rich fraction 3 taken off at the top of the removal column T1 can also he effected in only one heat exchanger. In this case, the substrearn 19 which is used as runback for the removal column T1 is taken off down-stream of this heat exchanger. Furthermore, the liquid C₅-hydrocarbon-rich fraction 14 is cooled to a temperature in the range from about −25 to −55° C. exclusively in the heat exchanger E1 and is then added to the substream 19 or introduced into the removal column T1 at a place between the top of the removal column T1 and the feed point for the hydrocarbon-rich fraction 2 to be liquefied.

The method of the invention for separating off heavy hydrocarbons from a hydrocarbon-rich fraction to be liquefied makes it possible, with a moderate outlay in terms of apparatus, for heavy hydrocarbons, in particular benzene, to be separated off from the hydrocarbon-rich fraction to be liquefied to a sufficient extent for precipitation. of solids to be avoided, without the energy consumption for the liquefaction process being appreciably increased. 

1. A method of separating heavy hydrocarbons, from a hydrocarbon-rich fraction to be liquefied, where the hydrocarbon-rich fraction is precooled before the heavy hydrocarbons are separated off, characterized in that a C₅-hydrocarbon-rich fraction is added to the hydrocarbon-rich fraction before the latter is precooled or upstream of the removal column serving to separate off the heavy hydrocarbons at least in such an amount, that freezing-out of the heavy hydrocarbons to be separated off is avoided.
 2. The method as claimed in claim 1, characterized in that the C₅-hydrocarbon-rich fraction to be added is at least partly formed from the hydrocarbon-rich fraction to be liquefied.
 3. The method as claimed in claim 1, characterized in that the content of C₂₊-hydrocarbons in the hydrocarbon-rich fraction to be liquefied is less than 15 mol %.
 4. The method as claimed in claim 1, characterized in that the C₅-hydrocarbon-rich fraction to be added contains from 50 to 100% by volume of i- and/or n-pentane.
 5. The method as claimed in claim 4, characterized in that the C₅-hydrocarbon-rich fraction to be added additionally contains propane, butane, hexane and/or higher hydrocarbons.
 6. The method as claimed in claim 1, characterized in that a substream of the hydrocarbon-rich fraction which has not been precooled is fed as heating medium into the removal column.
 7. The method as claimed in claim 1, characterized in that the C₅-hydrocarbon-rich fraction fed to the removal column is cooled to a temperature in the range from −100 to −130° C. and is fed into the removal column above the feed point for the hydrocarbon-rich fraction to be liquefied.
 8. The method as claimed in claim 1, characterized in that a substream of the hydrocarbon-rich fraction which has been freed of heavy hydrocarbons and has been liquefied is fed as runback into the removal column.
 9. The method as claimed in claim 1, characterized in that the hydrocarbon-rich fraction to be liquefied is precooled to a temperature in the range from −25 to −55° C. before the heavy hydrocarbons are separated off.
 10. The method as claimed in claim 1, wherein the heavy hydrocarbons are C₆₊-hydrocarbons and/or aromatic hydrocarbons.
 11. The method as claimed in claim 11, Wherein the heavy hydrocarbons are benzene.
 12. The method as claimed in claim 1, wherein the hydrocarbon-rich fraction is natural gas.
 13. The method of claim 1 wherein the heavy hydrocarbons are separated off in liquid form. 